Cellular communication systems are currently being developed and improved for machine type communication (MTC). In MTC, communication is characterized by lower demands on data rates than, e.g., mobile broadband. However, MTC has higher requirements on characteristics such as low cost device design, better coverage, and ability to operate for years on batteries without charging or replacing the batteries. At 3GPP RAN #70 meeting, a new work item named Narrowband Internet of Things (NB-IoT) was approved. The objective is to specify a radio access for cellular internet of things that addresses improved indoor coverage, support for massive numbers of low throughput devices, low delay sensitivity, ultra-low device cost, low device power consumption and (optimized) network architecture, while also maintaining backward compatibility with the current Long Term Evolution (LTE) radio access technology.
Three modes of operation are defined for NB-IoT: stand-alone, guard-band, and in-band. In stand-alone mode, the NB-IoT system is operated in dedicated frequency bands. For in-band operation, the NB-IoT system can be placed inside the frequency bands used by the current LTE system. In guard-band mode, the NB-IoT system can be placed in the guard band used by the current LTE system—that is, the frequencies below and above an LTE carrier that are within the allocated frequency band spectral mask, but are not used by the LTE carrier.
The NB-IoT utilizes the smallest allocable bandwidth unit in LTE: a Physical Resource Block (PRB), defined as 12 subcarriers by one slot (0.5 msec). With 15 KHz subcarrier spacing, the NB-IoT has a bandwidth of 180 KHz. The channel raster of the NB-IoT systems (like that of LTE) is on a frequency grid of 100 KHz. That is, NB-IoT devices attempt to access NB-IoT carriers in a step size of 100 KHz. For the standalone deployment, this is fine—the carriers can be placed on 100 KHz multiples. For the in-band deployment, only some of the PRBs in the LTE carrier are candidates for NB-IoT transmission—those that fall close to the 100 KHz raster. Similarly, in the guard-band deployment, an NB-IoT carrier may have to be spaced apart from the LTE carrier (in frequency) to be at or near a multiple of 100 KHz. This is not a problem with the wideband LTE carriers, which have correspondingly wide guard bands.
To achieve the coverage requirement, the transmission power of NB-IoT signals must be higher than that of the LTE carrier, e.g., by 6 dB. This is fine for stand-alone and in-band deployments. However, for guard-band NB-IoT, particularly in the lower-bandwidth LTE carriers, the requirement of 100 KHz raster spacing may move the NB-IoT carrier so far away from the LTE carrier—that is, closer to the edge of the allocated frequency band—that the 6 dB power boosting would cause the NB-IoT carrier to violate the strict requirement of the spectral mask. Hence, guard-band deployment is not possible in some of the lower bandwidth LTE carriers.
The Background section of this document is provided to place embodiments of the present invention in technological and operational context, to assist those of skill in the art in understanding their scope and utility. Approaches descried in the Background section could be pursued, but are not necessarily approaches that have been previously conceived or pursued. Unless explicitly identified as such, no statement herein is admitted to be prior art merely by its inclusion in the Background section.